alpha-Crystallin prevents irreversible protein denaturation and acts cooperatively with other heat-shock proteins to renature the stabilized partially denatured protein in an ATP-dependent manner

alpha-Crystallin, a major lens protein of approximate to 800 kDa with subun its of approximate to 20 kDa has previously been shown to act as a chaperon e protecting other proteins from stress-induced aggregation. Here it is dem onstrated that alpha-crystallin can bind to partially denatured enzymes at 42-43 degrees C and prevent their irreversible aggregation, but cannot prev ent loss of enzyme activity. However, the alpha-crystallin-bound enzymes re gain activity on interaction with other chaperones. The data indicate that the re-activated enzymes are no longer associated with the alpha-crystallin , and ATP is required for re-activation. When inactive luciferase bound to alpha-crystallin was treated with reticulocyte lysate, a rich source of cha perones, up to 60% of the original luciferase activity could be recovered. Somewhat less re-activation was observed when the alpha-crystallin-bound en zyme was treated with heat-shock protein (HSP)70, HSP40, HSP60 and an ATP-g enerating system. Similar results were also obtained with citrate synthase. The overall results suggest that alpha-crystallin acts to stabilize denatu ring proteins so that they can later be re-activated by other chaperones.